Frequency counters are digital instruments which measure the frequency of an input signal. Typically, the input signal operates a digital counter. In one counting technique, the count accumulated in the counter during a known time interval represents the frequency of the input signal. In an alternate counting technique, the time interval required to accumulate a predetermined count of input signal cycles is measured. The upper frequency limit for operation of digital circuitry is on the order of 500 MHz. In order to measure the frequencies of input signals above 500 MHz, it is necessary to down-convert the input signal to a lower frequency prior to frequency measurement. Down-conversion techniques include prescaling, heterodyne conversion, use of a transfer oscillator and harmonic heterodyne conversion. Of these techniques, harmonic heterodyne conversion is preferred because the number of microwave components is minimized, and measurements can be made up to 40 gigahertz.
The harmonic heterodyne converter utilizes a programmable local oscillator which provides a local oscillator frequency that is mixed with the input signal in a sampler. The output of the sampler is an intermediate frequency that is compatible with a digital frequency counter. The local oscillator frequency is varied until an output of the sampler falls within the intermediate frequency range. The frequency of the input signal is computed from the local oscillator frequency, the measured intermediate frequency and a harmonic number. The harmonic number identifies the harmonic of the local oscillator which is mixed with the input signal to produce the intermediate frequency. Since the intermediate frequency can be in either an upper sideband or a lower sideband, measurements are taken at two closely-spaced local oscillator frequencies in order to determine both the harmonic number and the sideband (upper or lower) in which the intermediate frequency is located.
The determination of harmonic number in the harmonic heterodyne technique is subject to various errors. Since the harmonic number can only be an integer, an error less than 0.5 is not significant, and rounding to the nearest integer produces a correct result. However, an error greater than 0.5 produces an erroneous harmonic number and causes a significant error in the measured microwave frequency.
A microwave frequency counter can be utilized to measure the frequency of both continuous wave and pulsed microwave signals. In the case of pulsed microwave signals having very short pulse durations, down conversion of the microwave signal results in an intermediate frequency pulse of only a few cycles. Such pulses are subject to measurement error due to the short observation time. Errors in harmonic number determination can also arise from frequency and amplitude variations of the input signal, and from other sources. It is desirable to provide a microwave frequency measurement technique wherein errors in harmonic number determination are minimized or eliminated.
It is a general object of the present invention to provide improved methods and apparatus for microwave frequency measurement.
It is another object of the present invention to provide methods and apparatus for accurate microwave frequency measurement.
It is a further object of the present invention to provide methods and apparatus for correcting errors in harmonic number determination in a harmonic heterodyne converter.